(1) Field of the Invention
This invention relates generally to an instant lighting lamp combining a miniature arc tube with a standby filament and is more particularly concerned with a ballasting arrangement to permit such a lamp to be used as a replacement for a conventional incandescent lamp.
(2) Description of the Prior Art
Electric arc lamps, such as the high pressure mercury vapor lamp or the metal iodide arc lamp related to it, are far more efficient light sources than the commonly used incandescent filament lamp. They have long been used for street lighting and in industrial applications. They have not been used at all in the home where most fixtures and lamps are designed to accommodate screw-in type incandescent lamps. Adapting these arc lamps, particularly in their smaller sizes, as direct replacements for incandescent lamps has become a serious energy saving goal.
The obstacle to be overcome in replacing the screw-in incandescent lamp with a small arc lamp is the ballasting circuit required to regulate the arc current being drawn from the fixed voltage AC power line. This circuit must be small and lightweight so that it can be integral to the light source package, and moreover, it must be simple and inexpensive so that the replacement lamp is affordable to the consumer. Most important, it should be energy-efficient so that the high efficiency of the arc lamp is not degraded by losses in the ballasting circuit.
As a replacement for an incandescent lamp in the home, two other pecularities of the arc lamp must be overcome to make the new device an acceptable light source. One is the slow arc warm-up during which time the light intensity only gradually increases, the other is the inability to hot restart, which means that a momentary shut-off of the arc by a power line interruption requires the lamp to first cool down, restart and then warm up again, during which time it does not produce much light. To remedy this unacceptable behavior, an auxiliary incandescent filament is included within the same glass jacket that encloses the small quartz arc tube. It produces light immediately upon turn-on while the arc lamp is warming up, and also comes on during any hot restart cycle so that there is always some light output produced.
Prior art ballast circuits have been designed to power a small direct current metal halide lamp. This lamp nominally contains a fill consisting of mercury, iodides of sodium and scandium, and argon gas. It requires a starting potential of several hundreds of volts to initiate ionization, several seconds of operation at about 200 V and a few tens of milliamperes to transfer from a glow to an arc discharge and full current for about a mninute warm-up during which time its potential drop rises from about 20 to 80 V. Such DC arc lamps are most simply operated in series with the auxiliary incandescent filament from a DC source obtained by rectifying and filtering the AC power line. In this way, the filament serves as a ballast and produces light during the arc warm-up. Separate circuitry must be used to turn on the filament during cool-down in a hot restart cycle. With this simple circuit, the voltage across the filament is equal to the difference between the rectifier output and the arc lamp voltage, and this difference decreases as the arc lamp warms up. After warm-up, little light is produced by the auxiliary filament, but current continues to flow through it, and its power dissipation is a significant source of inefficiency in this circuit.